Traditionally, an author that wishes to publish their paper, for example at a conference, must submit the paper for review. Under this system, the reviewers are not provided an incentive to provide fair and thorough reviews. There is often a conflict of interest since reviewers have also submitted papers for consideration. Often it is in the best interests of the reviewer to provide negative review of a paper, in order to maximise the chance that their own paper will be accepted for publication.
Also the traditional method does not attempt to keep reviewers accountable for their reviews. Further, there is little deterrent for preventing authors from submitting papers of poor quality for review.